Abstract

We report on the development of a superresolution four-wave mixing microscope with spatial resolution approaching 130 nm which represents better than twice the diffraction limit at 800 nm while retaining the ability to acquire materials- and chemical- specific contrast. The resolution enhancement is achieved by narrowing the microscope’s excitation volume in the focal plane through the combined use of a Toraldo-style pupil phase filter with the multiplicative nature of four-wave mixing.

Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Recall that in FWM three incident fields combine to generate a fourth field, thus two conventional and one phase filtered. Also of note, we do not account for the relative phase of the fields (intensity only) which will become increasingly important as the sample systems become microscopically complex/extended.

A two-dimensional (2D) spatial filtering of a 100 nm feature was carried out with the 2D filter function or kernel consisting of a spatially calibrated PSF.

Other (3)

Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Recall that in FWM three incident fields combine to generate a fourth field, thus two conventional and one phase filtered. Also of note, we do not account for the relative phase of the fields (intensity only) which will become increasingly important as the sample systems become microscopically complex/extended.

A two-dimensional (2D) spatial filtering of a 100 nm feature was carried out with the 2D filter function or kernel consisting of a spatially calibrated PSF.

Geometry of the annular phase masks used in programming the SLM. () A background mask which corrects the wavefront distortion in the optical system along with the series of annular masks used to generate the Toraldo phase filters. (B) A series of focal distributions (xy) resulting from application of the associated phase masks in (A) recorded by epi-two photon luminescence imaging of an isolated gold nanoparticle with 785 nm excitation. (C) shows a series of focal field calculations for the each of the annular phase masks in (A). The dimension is indicated by the λ: the wavelength of the incident beam. (D) xz focal field distributions resulting from application of the associated phase masks and recorded in a similar manner as the xy fields. (E) A series of xz focal field calculations.

(A) FWM microscopy images of Si nanowires on glass taken with 754 nm and 785 nm excitation beams. The arrow indicates the direction of the polarization and the dotted line indicates the region where the xz scans were recorded. (B) Shows the FWM PSF (xz) that results from two conventional beams. FWM PSF images that result from phase filtering with annular masks are shown in (C) for d = 0.14, (D) for d = 0.28, (E) for d = 0.42, (F) for d = 0.56, (G) for d = 0.70 and (H) for d = 0.83.

The FWM images of an isolated nanowire using conventional (A) and engineered PSFs corresponding to phase filters with annular diameters of (B) d = 0.14, (C) d = 0.28, (D) d = 0.35, (E) d = 0.42 and (F) d = 0.56. Insets in the lower-left corners correspond to simulated images using the calculated PSF for each phase filter. (G) The line profiles for the white dotted lines noted as (1, 2) in (A) and (D) are the average of three adjacent line scans: there was a mean variation of ~5% from line to line. (H) Simulated cross sections from the calculated images (inset) of a 100 nm feature using a conventional FWM PSF (black) and a SR-PSF (red). Note: The color scales are normalized, and the total signal strength relative to the conventional PSF case (maximum) is down a factor of 3.2 (minimum) for the d = 0.35 case.